Direct methanol fuel cell (DMFC)

ABSTRACT

The invention relates to a DMFC in which an evaporation apparatus is connected before the cell. The fuel, which is predominantly a methanol/water mixture with a possible admixture of inert gas, is variable in its composition, whereby the respective methanol/water and, if warranted, inert gas mixture, can be adjusted in load-dependent fashion. Moreover, the invention relates to a method for operating a DMFC apparatus in which the fuel is present in the anode chamber in gaseous form.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a direct methanol fuel cell (DMFC), anapparatus consisting of several DMFCs, and a method for operating DMFCapparatuses, with a high voltage efficiency and Faraday efficiency.

2. Description of the Related Art

The principle of the DMFC has been known since 1922; until now work hasconcentrated on the operation of the DMFC with liquid fuel. In the DMFC,methanol is taken as fuel; in earlier years alternatives to methanolwere tried, such as formic acid, formaldehyde or higher-chainedalcohols. The use of methanol thereby has the greatest technicalsignificance, for which reason also the name DMFC has become accepted.The operation of the DMFC with liquid fuel takes place at relatively lowtemperatures, and has the disadvantage that the conversion of themethanol takes place with a relatively poor voltage efficiency, due tokinetic inhibitions of the anode reaction.

The conversion of vaporized methanol is known from J.P.-22 34 359. Thewater for the moistening of the membrane and for the reaction sequenceis thereby supplied separately at the back side (i.e., at the cathodeside). The cathode-side supplying of the water has the disadvantage thatat higher current densities the electro-osmotic water transport, whichis proportional to the current, works against the water diffusionthrough the membrane. This causes increased water consumption, becausethe membrane has to be kept moist with additional water. Moreover, themetered addition of water does not take place in load-dependent fashionin this prior art.

A general problem in the operation of the DMFC remains the diffusion ofthe fuel methanol through the electrolytes to the cathode, where it isalso converted. The consequence of this, besides the loss of fuel(lowering of the Faraday efficiency), a reduction of the cell voltage(lowering of the voltage efficiency).

SUMMARY OF THE INVENTION

An object of the present invention is thus to provide a fuel cell and afuel cell apparatus, as well as a method for operating the apparatus, inwhich high voltage efficiency and high Faraday efficiency are realizedat high current densities. In addition, an object of the presentinvention is that a fuel cell, a fuel cell apparatus, and a method foroperating a fuel cell are provided that operate with low electro-osmoticwater loss in the cell and with the lowest possible water transportthrough the polymer electrolytes.

As Faraday efficiency, the degree of energy use is designated thatindicates what percentage of the fuel was actually converted at theanode.

As voltage efficiency, the ratio between cell voltage under currentloading and thermodynamic rest voltage is designated.

The general recognition of the invention is

First, that an increase of the Faraday efficiency is possible byminimization of the methanol diffusion inside the cell if the methanolis supplied in load-independent fashion and is consumed correspondinglyin the anode chamber. It is then not present in a concentration so highthat a large diffusion pressure toward the cathode arises.

Second, the invention is based on the recognition that the voltageefficiency can be improved by increasing the operating temperature,because this causes a minimization of the kinetic inhibition of theanode reaction. Moreover, the Faraday efficiency is also increased bythe load-dependent supplying of the reactands at a low current density.

Third, the problem of a too-high water transport through the polymerelectrodes can be reduced by the addition of an inert gas such as carbondioxide and/or nitrogen, because by this means the water content at theanode side of the DMFC is lowered, and less water is transported to thecathode.

The subject matter of the present invention is thus a DMFC comprising asupply duct and a waste removal duct for the fuel and the oxidantrespectively, a membrane electrode unit and bipolar plates, whereby anevaporating apparatus is connected before the supply duct for the fuelin such a way that, during the conversion, the fuel is present ingaseous form at the anode of the fuel cell. In addition, the subjectmatter of the present invention is a fuel cell apparatus that comprisesa cell stack of inventive fuel cells, an evaporation apparatus and, ifwarranted, up to three pumps (two dosing pumps for the supply ofmethanol and water and one pump that brings the CO₂ exhaust conducted inthe circuit back to the required excess pressure) in the supply line ofthe fuel as well as a CO₂ separator in the drainage line of the fuel,whereby, in the CO₂ separator which is connected after the fuel cellstack, the condensate of the gaseous fuel can be separated from thecarbon dioxide thermally or in some other way.

In addition, the subject matter of the present invention is a method foroperating a DMFC apparatus in which the fuel, consisting at least ofmethanol and water, is supplied to the anode in gaseous form.

The fuel of the inventive fuel cell can consist either of methanol onlyor of an arbitrary mixture of water and methanol. If the fuel consistsof an arbitrary mixture of water and methanol, then the concentrationeither of methanol or water can be adjusted in load-dependent fashionvia a dosing pump connected before the evaporation apparatus. The fuelcan thereby be introduced into the fuel cell with variable pressure, andan arbitrary mixture of inert carrier gas, such as CO₂, N₂, argon, etc.,can be mixed with it.

A preferred embodiment of the fuel cell provides that an inert gas, suchas e.g. carbon dioxide and/or nitrogen or the like, can be mixed withthe methanol/water mixture. By this means, the water content is reducedat the anode side of the DMFC, and less water is transported through thepolymer electrodes to the cathode side.

The degree of moistness x_(f)=V_(nW)/V_(n), where V_(nW)=water vaporvolume under normal conditions; V_(n)=total volume under normalconditions, can be adjusted arbitrarily by means of the inert gas.Degrees of moistness greater than 70%, preferably between 80 and 90%,prove useful, because then the polymer membrane is not yet dried out.The degree of moistness will be as high as possible so that the energyexpenditure for the gas transport remains as low as possible. The degreeof moistness also depends on the operating temperature of the DMFC. Thehigher this is, the higher the degree of moistness must also be, sincethe water content in the membrane decreases rapidly at temperaturesabove 100° C. The degree of moistness x_(f) (in relation to the volume)is defined as follows:

 x _(f) =V _(nW) /V _(n) =V _(nW)/(V _(nW) +V _(nL))=p _(W) /p

V_(nL)=dry gas volume under normal conditions, i.e. the volume ofgaseous methanol, with or without inert gas additive;

p_(W)=water vapor partial pressure

p=total pressure

The inventive fuel cell apparatus preferably consists of a cell stack ofinventive fuel cells, but it can also be constructed of various types offuel cells in combination. The evaporation apparatus and, if warranted,one or two dosing pumps that supply the fuel or the water inload-dependent fashion are thereby integrated into the supply line ofthe fuel to the cell stack.

In an anode circuit, in the CO₂ separator the CO₂ that has arisen isseparated from the exhaust gas, which is rich in unconsumed methanol.The exhaust gas is then present in condensed form and can be supplied inthe circuit, i.e. can be introduced into the evaporation apparatus. Inaddition, a part of the separated carbon dioxide can likewise beconducted in the circuit via an excess pressure pump that also regulatesthe amount of added inert gas.

Anode circuit means that the fuel, methanol or methanol/water mixture,respectively with or without inert gas additive, is conducted past theanode in a circular closed system, whereby additional fuel is suppliedto the system as needed, and gaseous reaction product is separated outfrom the system.

The unconsumed fuel contained in the fuel exhaust gas is first condensedor cooled using heat, and is then introduced again into the supply lineor into the evaporation apparatus. The load-dependent controlling of thedosing pumps that regulate the inflow of water/methanol into theevaporation apparatus must thereby be constructed in such a way that thechanges in concentration of the methanol/water mixture in theevaporation apparatus due to the supplying from the exhaust gas aretaken into account.

The unconsumed fuel from the fuel exhaust gas is separated from thecontained carbon dioxide physically, or in some circumstances, alsochemically in the heat exchanger or CO₂ separator. Physical separationthereby means that the separation takes place via the various physicalcharacteristics of the substances (such as density, boiling point,etc.). Chemical separation is also conceivable, including means that theCO₂ is chemically bound and precipitated e.g. as carbonate (not veryuseful energetically, due to the high mass of the resulting carbonate,but alternative chemical methods can be discussed).

As DMFC the direct methanol fuel cell is designated that consists of ananode, a cathode and a suitable electrolyte, in analogy to the generalprinciple of electrochemical energy converters. In general, theelectrodes are contacted at the back side, i.e. with the side facingaway from the electrolyte, through a current collector which has inaddition the task of gas distribution or, respectively, reactanddistribution. As a result of the type of electrolytes used, there resultvarious possibilities for the realization of a DMFC. In the context ofthe present invention, preferred acid electrolytes, and thereby inparticular acid solid electrolytes, are treated. In general,proton-conducting polymers (electrolyte membranes), which are stableunder the corresponding operating conditions, are thereby suitable. Asan example, NAFION (registered trademark) is hereby mentioned as asuitable polymer. As a further electrolyte apart from those mentioned,those based on inorganic systems are hereby also mentioned, such as tinphosphates or electrolytes based on siloxane frameworks.

As current collectors, materials based on carbon, e.g. carbon fiberpaper or tissue, are standardly used. As catalysts, at the anode sideplatinum/ruthenium alloys are used with first priority; at the cathodeside pure platinum is mostly used. In the realization of a fuel cellapparatus, such as e.g. a battery, in order to achieve higher voltagesthe individual cells are connected in series in bipolar fashion. Thebipolar plates required therefor can be made of graphite, metallic orother electrically conductive and corrosion-resistant materials. Thebipolar plates simultaneously take over the task of reactand supplying.They are thus structured with corresponding ducts, if necessary.

According to the boiling point of the mixture, the operation of the DMFCcan take place at temperatures between 60 and 160° C. The operatingtemperature will preferably fall into a range from 100 to 150° C.;typically it is between 120 and 130° C. Correspondingly, methanol oralso corresponding methanol/water mixtures are heated above the boilingpoint and supplied to the cell in gaseous form. The system pressure isthereby adjusted so that it corresponds to the equilibrium pressure ofthe methanol/water mixture at the temperature of the fuel cells. In theanode chamber of the DMFC, the vapor is thus in a saturated state. Bymeans of this vaporized supplying of the reactands, the electro-osmoticwater transport is minimized, because the quantity of water at the anodeis greatly reduced. In the context of the present application, the terms“fuel,” “methanol” and “mixture of water and methanol” always designatea vaporized fuel that contains an indeterminate quantity of inert gas(i.e., from 0% up to a degree of moistness of almost 100). In the caseof CO₂ as an inert gas, it can be a part of the anode exhaust gas thatis again brought to the required excess pressure via a pump and acorresponding control valve (see also FIG. 2) and is conducted in thecircuit.

As stated, a methanol/water mixture or pure ethanol, with or withoutinert gas additive, is used as fuel; however, the invention is not to belimited thereto, if the electrochemical oxidability of otherwater-soluble organic molecules turns out to be technically profitable.As stated, the fuel is conducted in the circuit via a carbon dioxideseparator connected to the exhaust gas line of the fuel cell, which hasat the same time the function of separating the resulting carbon dioxidefrom the rest of the exhaust gas.

As an oxidant, either pure oxygen or air or arbitrary mixtures of thesecomponents are designated, whereby the oxidant of the cathode ispreferably supplied in a quantity leaner than stoichiometric.

A particular problem of the DMFC is the search for suitable anodematerials for the oxidation of the fuel. Thus, besides the namedplatinum/ruthenium alloys, according to the state of research, variousanode materials and catalysts can be used on the anode according to theinvention. For example, it is hereby mentioned that under somecircumstances it is possible to bring about a slight additionalimprovement of the activity of the anode in relation to the binarysystem platinum/ruthenium by adding a third component, such as tin ornickel, to the alloy. The invention is not to be limited to noble metalsas catalysts and anode materials or, respectively, cathode materials;rather, catalysts free of noble metals are also conceivable.

The concentration of methanol in the fuel cell mixture in relation tothe unvaporized liquid state can be between 0.05 and 5 mol/l. Aconcentration between 0.5 and 1.5 mol/l is thereby particularlypreferred.

As a further operating parameter the pressure is hereby mentioned, whichcan be between normal pressure and a slight excess pressure and partialvacuum. The above definitions hold for the specification, theexplanations of the Figures and the claims.

Other objects and advantages of the present invention will becomeapparent from reading the following detailed description and appendedclaims, and upon reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is explained in more detail on the basisof two Figures.

FIG. 1 shows a block switching diagram of an inventive fuel cellapparatus.

FIG. 2 likewise shows a block switching diagram of an inventive fuelcell apparatus, in which, however, the fuel cell is conducted in thecircuit via a CO₂ separator connected to the fuel cell.

It should be understood that the drawings are not necessarily to scaleand that the embodiments are sometimes illustrated by graphic symbols,phantom lines, diagrammatic representations and fragmentary views. Incertain instances, details which are not necessary for an understandingof the present invention or which render other details difficult toperceive may have been omitted. It should be understood, of course, thatthe invention is not necessarily limited to the particular embodimentsillustrated herein.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 shows a fuel cell apparatus that can be operated both with amethanol/water mixture and also with pure methanol as fuel. Theapparatus has dosing pumps 3 and 3′, which regulate the supply ofmethanol or/and water in load-dependent fashion via a control system.From left to right, FIG. 1 shows, first, the two containers 1 and 2 inwhich water and methanol are contained. From these supply containers 1and 2, the components of the liquid fuel, i.e. water and methanol, flowinto the dosing pumps 3 and 3′, which respectively regulate the flowspeed of the liquids. In the case of the supply container 1, which isfor example the water container, the quantity of water metered inload-dependent fashion in the dosing pump 3 flows into the evaporationapparatus 4 via the line 11. Likewise, a particular quantity of methanolflows from the supply container 2 via the dosing pump 3′ and the line 12into the same evaporation apparatus 4. In the evaporation apparatus 4,both liquids are heated over the boiling point, and via the line 13 thevapor mixture that results in the evaporation apparatus is introducedinto the fuel cell stack 5. There it is conducted into the respectiveanode chambers of the individual fuel cells via a supply duct. Via theline 15, the consumed fuel exhaust, enriched with CO₂, again leaves thefuel cell stack and flows into the carbon dioxide separator or heatexchanger 6, in which it is again condensed out if necessary, using theheat energy. The resulting CO₂ can there be separated from the exhaustgas/condensate. Via the line 14, parallel to the line 13, the fuel cellis supplied with oxidant at the cathode side. The oxidant exhaust gasleaves the cell stack again via the line 16 and is conducted into theheat exchanger 7.

FIG. 2 shows a similar block switching diagram of an inventive fuel cellapparatus, with the difference that the heat exchanger or carbon dioxideseparator 6, connected to the fuel exhaust gas line from the fuel cellstack, is connected with the evaporation apparatus 4 via the line 8. Viathe line 8, the fuel exhaust gas, condensed out or cooled in the heatexchanger or carbon dioxide separator 6 if warranted, now again flowsinto the evaporation apparatus 4, where it is again supplied to the fuelcell via the line 13. Via a second line 18, the deposited CO₂ islikewise conducted from the heat exchanger 6 into the evaporationapparatus 4. In the line 18 there is a pump 19 through which the CO₂ isagain brought to the required excess pressure.

As stated, the composition of the fuel mixture is oriented according tothe respective load of the fuel cell stack and the predetermination ofthe degree of moisture. Via a regulating mechanism that compares thepredetermined water/methanol concentrations, given in terms of the load,as a target value with the actual value of the mixture given in the line13, the performance of the dosing pumps 3 and 3′ is adjusted. Theaddition of inert gas is likewise controlled via a regulating mechanismthat compares the degree of moisture in the line 13, as an actual value,with a predetermined degree of moisture as a target value. The inert gasmay be added by way of a line 21 directly to the evaporator 4 (FIG. 1)or to the pump 19 which precedes the evaporator 4 (FIG. 2). A pump 22(FIG. 1) may also be included in the inert gas supply line 21. Thisembodiment of the invention thus enables an optimization of the Faradayefficiency.

A fuel cell, in particular a battery consisting of the inventive fuelcells, works with a voltage efficiency and Faraday efficiency that isessentially increased in relation to the prior art. In addition, bymeans of the vaporized presence of the reactants the “flooding” of thecathode is prevented, or at least is largely forced back. In this case,“flooded” means that methanol and water flow into the working layer ofthe cathode and the hydrophocity of the gas transport pores decreases,so that they are full of reaction water, thus hindering the transport ofoxygen and causing the cell voltages to collapse.

The increasing of the Faraday efficiency is thereby achieved mainly bymeans of the minimization of the methanol diffusion through themembrane. Via the dosing pump 3, controlled in load-dependent fashion,only as much methanol is respectively evaporated in the evaporationapparatus 4 as is required in the current operating state of fuel cellstacks. “Fuel requirement” is thereby defined as a load-dependent targetvalue that by means of the Faraday equivalents of the reaction and anoperationally caused bandwidth, which bandwidth is preferably present inan excess. The methanol concentration is thus variably adjustable at theanode, following the change of load, and extreme operating parameters(such as standby and full load) can also be adjusted in a state that isoptimally close to the limiting diffusion current (i.e., still withmaximum performance but close to the limiting diffusion current alongthe characteristic line in the voltage/current diagram). Themodification of the methanol concentration in the gas mixture must notbe regulated via the water supply or the pressure; rather, it can ofcourse also be controlled via the addition of an inert carrier gas.

For the adjustment of the dosing pumps, no extra measurement of theactual value of the concentration of methanol or fuel prevailing in thecell need take place, because the dosing pump can be adjusted inload-dependent fashion, and the consumption of methanol, and therebyalso the quantity still contained in the recycled exhaust gas, can becalculated via the current curve.

An additional monitoring determination of the actual value of themethanol concentration in the gaseous fuel mixture can however also forexample take place in the supply line 13 from the evaporation apparatusto the fuel stack, or in the evaporation apparatus itself. If thedetermination of the actual value takes place in the evaporationapparatus, the container of the evaporation apparatus must be selectedwith respect to its dimension in such a way that a complete evaporationis ensured in all conceivable operational states, and thus no change inconcentration due to condensation results. As a rule, however, amonitoring determination of the water-methanol mixture ratio will takeplace, if at all, as far as possible in the direct supply line to thesupply duct of the fuel cell stack.

According to the invention, a simpler construction of the cell isrealized in that the water is supplied at the anode side and not, as inthe cited prior art, at the cathode side.

A battery consisting of inventive fuel cells is, among other things,conceivable for use in mobile energy supply, such as for example in anautomobile. However, it is also conceivable in larger stationary energysupply installations, such as for example in power stations or for thesupply of electrical power and heat to residential buildings or officebuildings.

From the above description, it is apparent that the objects of thepresent invention have been achieved. While only certain embodimentshave been set forth, alternative embodiments and various modificationswill be apparent from the above description to those skilled in the art.These and other alternatives are considered equivalents and within thespirit and scope of the present invention.

What is claimed is:
 1. A direct methanol fuel cell system comprising: awater supply line connected to an evaporation apparatus, the watersupply line comprising a first dosing pump for regulating a flow ofwater to the evaporation apparatus, a methanol supply line connected tothe evaporation apparatus, the methanol supply line comprising a seconddosing pump for regulating a flow of methanol to the evaporationapparatus, the evaporation apparatus being connected to a fuel cellstack by a supply duct, the fuel cell stack being connected to a fuelwaste removal duct and an oxidant waste removal duct, the fuel cellstack further comprising a plurality of fuel cells each comprising, amembrane-electrode unit and bipolar plates, the fuel waste removal ductbeing connected to a separator for separating gaseous carbon dioxidefrom fuel waste, the separator being connected to the evaporationapparatus by a supply line for transmitting carbon dioxide from theseparator to the evaporation apparatus, the evaporation apparatusvaporizing water and methanol and supplying gaseous water, methanol andcarbon dioxide to the fuel cell stack through the supply duct, wherebythe concentrations of methanol, water and/or carbon dioxide in thesupply duct can be adjusted in load-dependent fashion adjusting thedosing pumps.
 2. The direct methanol fuel cell system of claim 1 whereina degree of moistness at the membrane-electrode unit is greater than70%.
 3. The direct methanol fuel cell system claim 1 wherein theseparator separates unconsumed fuel from the reaction product carbondioxide.
 4. The direct methanol fuel cell system of claim 1 wherein theseparator further comprises a heat exchanger.
 5. A direct methanol fuelcell system comprising: a water supply line connected to an evaporationapparatus, the water supply line comprising a first adjustable valve forregulating a flow of water to the evaporation apparatus, a methanolsupply line connected to the evaporation apparatus, the methanol supplyline comprising a second adjustable valve for regulating a flow ofmethanol to the evaporation apparatus, the evaporation apparatus beingconnected to a fuel cell stack by a supply duct, the fuel cell stackbeing connected to a fuel waste removal duct and an oxidant wasteremoval duct, the fuel cell stack further comprising a plurality of fuelcells each comprising, a membrane-electrode unit and bipolar plates, thefuel waste removal duct being connected to a separator for separatinggaseous carbon dioxide from fuel waste, the fuel waste removal ductcomprising a third adjustable valve for regulating a flow of carbondioxide and fuel waste to the separator, the separator being connectedto the evaporation apparatus by a supply line for transmitting carbondioxide from the separator to the evaporation apparatus, the evaporationapparatus vaporizing water and methanol and supplying gaseous water,methanol and carbon dioxide to the fuel cell stack through the supplyduct, whereby the concentrations of methanol, water and/or carbondioxide in the supply duct can be adjusted by adjusting the first,second and third adjustable valves.
 6. The direct methanol fuel cellsystem of claim 5 wherein a degree of moistness at themembrane-electrode unit is greater than 70%.
 7. The direct methanol fuelcell system of claim 5 wherein the separator separates unconsumed fuelfrom the reaction product carbon dioxide.
 8. The direct methanol fuelcell system of claim 5 wherein the separator further comprises a heatexchanger.
 9. A method for the operation of a direct methanol fuel cellapparatus including a fuel cell connected to a supply duct, the fuelcell also being connected to a fuel waste removal duct, the fuel cellfurther comprising a membrane electrode unit and bipolar plates, themethod comprising: controlling amounts of gaseous methanol, water and/oran inert gas supplied to the fuel cell through the supply duct in aload-dependent fashion, separating carbon dioxide from a waste fuelflowing through the fuel waste removal duct, and combining at least aportion of the carbon dioxide with the methanol, water and inert gasbeing supplied to the fuel cell through the supply duct.
 10. A directmethanol fuel cell system comprising: a water supply line connected toan evaporation apparatus, the water supply line having a first dosingpump for regulating a flow of water to the evaporation apparatus, amethanol supply line connected to the evaporation apparatus, themethanol supply line having a second dosing pump for regulating a flowof methanol to the evaporation apparatus, an inert gas supply lineconnected to the evaporation apparatus, said inert gas supply linehaving a third dosing pump for regulating a flow of inert gas to theevaporation apparatus, the evaporation apparatus being connected to afuel cell stack by a supply duct, the fuel cell stack being connected toa fuel waste removal duct and an oxidant waste removal duct, the fuelcell stack further comprising a plurality of fuel cells each comprisinga membrane-electrode unit and bipolar plates, and a separator forseparating gaseous carbon dioxide from fuel waste being connected to thefuel waste removal duct and having an outlet connected to the thirddosing pump for transmitting carbon dioxide from the separator to theevaporation apparatus to be combined with the water and methanol, theevaporation apparatus vaporizing water and methanol and supplyinggaseous water, methanol and inert gas to the fuel cell stack through thesupply duct, whereby concentrations of methanol, water and/or inert gasin the supply duct can be adjusted by adjusting the first, second andthird dosing pumps.
 11. The direct methanol fuel cell system of claim10, wherein the separator separates unconsumed fuel from the reactionproduct carbon dioxide and has a line extending to the evaporationapparatus to supply the unconsumed fuel to the evaporation apparatus.12. The direct methanol fuel cell system of claim 10, wherein theseparator comprises a heat exchanger.